Vibration is a common issue on ships that can cause increased stresses, energy losses, wear, and fatigue. There are two main types of vibration - free vibration that occurs when a system is set in motion and allowed to vibrate at its natural frequency, and forced vibration caused by an external alternating force. Ship vibrations are typically from engines, propellers, and wave/sea motion. Reducing vibration through proper design and maintenance can increase machinery life by lowering dynamic forces and vibration amplitudes. Vibration monitoring is used to detect potential issues early through measuring characteristics like frequency, displacement, velocity, and acceleration.

1. Question:
State applications of polymers and other non metallic materials on board ship
Due date: 31st December 2008 (before 1700 hrs)
Assessment: 5%

2. Vibration is the motion of a particle or a body or a system of
connected bodies displaced from a position of equilibrium.
Most vibrations are undesirable in machines and structures because
they produce:
• increased stresses
• energy losses
• cause added wear
• increase bearing loads
• induce fatigue
• create passenger discomfort in vehicles
• absorb energy from the system

3. Free vibration occurs when a mechanical system is set off with an
initial input and then allowed to vibrate freely. Examples of this type
of vibration are pulling a child back on a swing and then letting go or
hitting a tuning fork and letting it ring. The mechanical system will
then vibrate at one or more of its "natural frequencies" and damp
down to zero.
Forced vibration is when an alternating force or motion is applied
to a mechanical system. Examples of this type of vibration include a
shaking washing machining due to an imbalance, transportation
vibration or the vibration of a building during an earthquake.
In forced vibration the frequency of the vibration is the frequency of
the force or motion applied, with order of magnitude being
dependent on the actual mechanical system.

4. Components in a vibrating system have three properties of
interest. They are:
• mass (weight)
• elasticity (springiness)
• damping (dissipation)
Most physical objects have all three properties, but in many
cases one or two of those properties are relatively insignificant
and can be ignored
For example, the damping of a block of steel, or in some cases,
the mass of a spring).

5. The property of mass (weight) causes an object to resist acceleration.
It also enables an object to store energy, in the form of velocity
(kinetic) or height (potential).
The property of elasticity enables an object to store energy in the
form of deflection. A common example is a spring, but any piece of
metal has the property of elasticity. That is, if you apply two equal
and opposite forces to opposite sides of it, it will deflect. Sometimes
that deflection can be seen; sometimes it is so small that it can't be
measured with a micrometer. The size of the deflection depends on
the size of the applied force and the dimensions and properties of the
piece of metal. The amount of deflection caused by a specific force
determines the "spring rate" of the metal piece. Note that all metals
(in the solid state) have some amount of elasticity.

6. The property of damping enables an object to DISSIPATE energy,
usually by conversion of kinetic (motion) energy into heat energy.
The misnamed automotive device known as a "shock absorber" is a
common example of a damper. If you push on the ends of a fully
extended "shock absorber" (so as to collapse it) the rod moves into the
body at a velocity related to how hard you are pushing. Double the
force and the velocity doubles. When the "shock" is fully collapsed,
and you release your hand pressure, nothing happens (except maybe
you drop it). The rod does not spring back out. The energy (defined as
a force applied over a distance) which you expended to collapse the
damper has been converted into heat which is dissipated through the
walls of the shock absorber.

7. The resonant frequency, ωn of an object (or system) is the frequency at
which the system will vibrate if it is excited by a single pulse. As an
example, consider a diving board. When a diver bounces on the end of
the board and commences a dive, the board will continue to vibrate up
and down after the diver has left it. The frequency at which the board
vibrates is it’s resonant frequency, also known as it’s natural frequency.
Another example is a tuning fork. When struck, a tuning fork "rings" at
it’s resonant frequency. The legs of the fork have been carefully
manufactured so as to locate their resonant frequency at exactly the
acoustic frequency at which the fork should ring.
k
ωn = where "k" is the appropriate
elasticity value and "m" is the
m appropriate mass value.

8. A waveform is a pictorial representation of a vibration.
Example:

9. VIBRATION AS AN INDICATOR OF MACHINERY
CONDITION
• Machines of some kind are used in nearly every aspect of our daily lives
• How many times have you touched a machine to see if it was "running
right"? With experience, you have developed a "feel" for what is normal
and what is abnormal in terms of machinery vibration.
• Even the most inexperienced driver knows that something is wrong when
the steering wheel vibrates or the engine shakes. In other words, it's
natural to associate the condition of a machine with its level of vibration.
• Of course, it's natural for machines to vibrate. Even machines in the best of
operating condition will have some vibration because of small, minor
defects. Therefore, each machine will have a level of vibration that may be
regarded as normal or inherent. However, when machinery vibration
increases or becomes excessive, some mechanical trouble is usually the
reason. Vibration does not increase or become excessive for no reason at
all. Something causes it - unbalance, misalignment, worn gears or
bearings, looseness, etc.

10. • When a machine fails or breaks down, the consequences can range from
annoyance to financial disaster, or personal injury and possible lose of
life
• For this reason, the early detection, identification and correction of
machinery problems is paramount to anyone involved in the maintenance
of industrial machinery to insure continued, safe and productive
operation
WHAT IS VIBRATION?
Vibration can be defined as simply the cyclic or oscillating motion
of a machine or machine component from its position of rest.

11. WHAT CAUSES VIBRATION?
Forces generated within the machine cause vibration. These forces may:
3.Change in direction with time, such as the force generated by a rotating
unbalance.
2. Change in amplitude or intensity with time, such as the unbalanced
magnetic forces generated in an induction motor due to unequal air gap
between the motor armature and stator (field).
3. Result in friction between rotating and stationary machine components in
much the same way that friction from a rosined bow causes a violin string to
vibrate.
4. Cause impacts, such as gear tooth contacts or the impacts generated by the
rolling elements of a bearing passing over flaws in the bearing raceways.
5. Cause randomly generated forces such as flow turbulence in fluid-handling
devices such as fans, blowers and pumps; or combustion turbulence in gas
turbines or boilers.

12. Some of the most common machinery problems that cause vibration include:
2.Misalignment of couplings, bearings and gears
2. Unbalance of rotating components
3. Looseness
4. Deterioration of rolling-element bearings
5. Gear wear
6. Rubbing
7. Aerodynamic/hydraulic problems in fans, blowers and pumps
8. Electrical problems (unbalance magnetic forces) in motors
9. Resonance
10. Eccentricity of rotating components such as "V" belt pulleys or gears

13. VIBRATION AND MACHINE LIFE
Question: "Why worry about a machine's vibration?"
Once a machine is started and brought into service, it will not run
indefinitely. In time, the machine will fail due to the wear and ultimate
failure of one or more of its critical components. And, the most
common component failure leading to total machine failure is that of
the machine bearings, since it is through the bearings that all machine
forces are transmitted.
Answer :
1. Increased dynamic forces (loads) reduce machine life.
2. Amplitudes of machinery vibration are directly proportional to
the amount of dynamic forces (loads) generated.
3. Logically then, the lower the amount of generated dynamic
forces, the lower the levels of machinery vibration and the longer
the machine will perform before failure.

14. When the condition of a machine deteriorates, one of two (and possibly
both) things will generally happen:
3.The dynamic forces generated by the machine will increase in
intensity, causing an increase in machine vibration.
Wear, corrosion or a build-up of deposits on the rotor may
increase unbalance forces. Settling of the foundation may increase
misalignment forces or cause distortion, piping strains, etc.
2. The physical integrity (stiffness) of the machine will be
reduced, causing an increase in machine vibration.
Loosening or stretching of mounting bolts, a broken weld, a
crack in the foundation, deterioration of the grouting, increased bearing
clearance through wear or a rotor loose on its shaft will result in
reduced stiffness to control even normal dynamic forces.

15. VIBRATION AS A PREDICTIVE MAINTENANCE TOOL
There are many machinery parameters that can be measured and
trended to detect the onset of problems. Some of these include:
1. Machinery vibration
2. Lube oil analysis including wear particle analysis
3. Ultrasonic (thickness) testing
4. Motor current analysis
5. Infrared thermography
6. Bearing temperature
In addition, machinery performance characteristics such as flow rates
and pressures can also be monitored to detect problems. In the case of
machine tools, the inability to produce a quality product in terms of
surface finish or dimensional tolerances is usually an indication of
problems. All of these techniques have value and merit.

16. A vibration predictive maintenance program consists of three logical
steps:
1. DETECTION
measuring and trending vibration levels at marked locations on each
machine included in the program on a regularly scheduled basis.
Typically, machines are checked on a monthly basis.
However, more critical machines may be checked more frequently or,
perhaps, continually with permanently installed on-line vibration
monitoring systems. The objective is to reveal significant increases in a
machine's vibration level to warn of developing problems.

17. 2. ANALYSIS
Once machinery problems have been detected by manual or on- line
monitoring, the obvious next step is to identify the specific
problem(s) for scheduled correction. This is the purpose of
vibration analysis – to pinpoint specific machinery problems by
revealing their unique vibration characteristics.
3. Correction
Once problems have been detected and identified, required
corrections can be scheduled for a convenient time. Of course, in the
meantime, any special requirements for repair personnel (including
outside repair facilities), replacement parts and tools can be arranged
in advance to insure that machine downtime is kept to an absolute
minimum.

18. CHARACTERISTICS OF VIBRATION
Vibration is simply defined as "the cyclic or oscillating motion of a
machine or machine component from its position of rest or its
'neutral' position.“
Whenever vibration occurs, there are actually four (4) forces
involved that determine the characteristics of the vibration. These
forces are:
1. The exciting force, such as unbalance or misalignment.
2. The mass of the vibrating system, denoted by the symbol (M).
3. The stiffness of the vibrating system, denoted by the symbol (K).
4. The damping characteristics of the vibrating system, denoted by
the symbol (C).
The exciting force is trying to cause vibration, whereas the stiffness,
mass and damping forces are trying to oppose the exciting force and
control or minimize the vibration.

19. The characteristics needed to define the vibration include:
2.Frequency
The amount of time required to complete one full cycle of the
vibration is called the period of the vibration.
5.Displacement
The total distance traveled by the vibrating part from one extreme
limit of travel to the other extreme limit of travel. This distance is
also called the "peak-to-peak displacement".
9.Velocity
The time required to achieve fatigue failure is determined by both
how far an object is deflected (displacement) and the rate of deflection
(frequency). If it is known how far one must travel in a given period of
time, it is a simple matter to calculate the speed or velocity required.
Thus, a measure of vibration velocity is a direct measure of fatigue.

20. 1. Acceleration
Acceleration is the rate of change of velocity.
4. Phase
With regards to machinery vibration, is often defined as "the position
of a vibrating part at a given instant with reference to a fixed point or
another vibrating part".
Another definition of phase is: "that part of a vibration cycle where
one part or object has moved relative to another part".

21. Vibration in Ship
• Vibration from engines, propellers, etc., tends to cause strains in
the after part of the ship.
• It is resisted by special stiffening of the cellular double bottom
under engine spaces and by local stiffening in the region of the
stern and after peak.

22. Stresses in Ships
 These may be divided into two classes:
2. Structural – affecting the general structure and shape of the ship.
3. Local – affecting certain localities only.
 A ship must be built strongly enough to resist these stresses,
otherwise they may cause strains.
 It is, therefore, important that we should understand the principal
ones and how they caused and resisted.
Principal Structural Stresses
 Hogging and Sagging; Racking; effect of water pressure; and
drydocking.
Principal Local Stresses
 Panting; Pounding; effect of local weights and vibration.

23. Hogging and Sagging
• These are longitudinal bending stresses, which may occur when a
ship is in a seaway, or which may be caused in loading her.
• Figure 2 shows how a ship may be hogged and Figure 3 how she
may be sagged by the action of waves.
Figure 2
Figure 3
• When she is being loaded, too much weight in the ends may cause
her to hog, or if too much weight is placed amidships, she may
sag.

24. Racking
• Figure 4 shows how a ship may be “racked” by wave action, or
by rolling in a seaway.
• The stress comes mainly on the corners of the ship, that is, on the
tank side brackets and beam knees, which must be made strong
enough to resist it.
• Transverse bulkheads provide very great resistance to this stress.
Effect of Water Pressure
 Water pressure tends to push-in the sides and bottom of the ship.
 It is resisted by bulkheads and by all transverse members (Fig. 5).
Figure 4 Figure 5

25. Panting
• Panting is an in and out motion of the plating in the bows of a ship
and is caused by unequal water pressure as the bow passes
through successive waves.
• Fig. 6 illustrates how it is caused.
• It is greatest in fine bowed ships.
• For the means adopted to resist it,
see “Peak Tanks.”
Figure 6

26. Pounding
 When a ship is pitching, her bows often lift clear of the water and
then come down heavily, as shown in Fig. 7.
Figure 7
 This is known as “pounding” and occurs most in full-bowed ships.
 It causes damage to connections and riveting in the three strakes
of plating next to the keel and in the general girder-work of the
inner bottom just abaft the collision bulkhead.
 For the strengthening to resist pounding see “Cellular Double
Bottoms.”

27. Local Weights
• Local strengthening is introduced to resist stresses set up local
weights in a ship, such as engines.
• This is also done where cargoes imposing extraordinary local
stresses are expected to be carried.
Drydocking
 It can be seen from Fig. 8 that a ship, when
in drydock and supported by the keel blocks,
will have a tendency to sag at the bilges.
 In modern ships of normal size, the cellular
double bottom is strong enough to resist this
stress without any further strengthening.
 It is worth noting that if sagging does occur, Figure 8
it can always be remedied by the use of bilge
blocks.